1. Field of the Invention
The present invention relates to a ferromagnetic tunnel junction structure constituted of a tunnel barrier layer and two ferromagnetic layers that sandwiches the tunnel barrier layer, and application of the same structure to a magnetoresistive effect device and to spintronics devices. Priority is claimed on Japanese Patent Application No. 2012-039582 filed on Feb. 27, 2012, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, giant magnetoresistance (GMR) effect device constituted of multilayers of ferromagnetic layer/nonmagnetic metal layer, and ferromagnetic tunnel junction (MTJ) device constituted of ferromagnetic layer/insulation layer (tunnel barrier layer, barrier layer)/ferromagnetic layer have been paid much attentions to as novel devices for magnetic field censors and nonvolatile random access magnetic memories (MRAM). Known types of GMR includes current-in-plane type GMR(CIP-GMR) in which the electric current flows in the film plane, and current-perpendicular-to-plane type GMR(CPP-GMR) in which the electric current flows in a direction perpendicular to the film plane. The principle of GMR mainly depends on spin-dependent scattering in the interface between the magnetic layer and the nonmagnetic layer, and includes contribution from spin-dependent scattering (bulk-scattering) in the magnetic body.
Since the contribution of bulk-scattering is expected in the CPP-GMR, CPP-GMR is generally constituted larger than the CIP-GMR. A spin-valve type device is used as the GMR device, where spins in a ferromagnetic layer are fixed by an antiferromagnetic layer that is close to the ferromagnetic layer.
On the other hand, so-called tunnel magnetoresistance (TMR) effect is obtained in the MTJ device, where intensity of tunnel current in the direction perpendicular to the layer plane are made different in the two ferromagnetic layers by controlling magnetization of the two ferromagnetic layers in parallel or in anti-parallel.
The TMR value in the tunnel junction depends on spin polarization P in the interface between the ferromagnetic body and insulator, and is generally given by the below described formula 1, where P1 and P2 denote spin polarizations of two ferromagnetic bodies.TMR=2P1P2/(1−P1P2)  formula 1
Here, the spin polarization of a ferromagnetic body satisfies 0<P≦1. Conventionally, Al oxide (AlOx) film of amorphous structure and MgO film with (001) plane orientation have been used as the barrier. In the former case, the film is formed by oxidation (for example, by plasma oxidation) of Al metal film formed, for example, by sputtering. It is widely known that such a film has an amorphous structure (for example, J. S. Moodera et al., Phys. Rev. Lett. 74, 3273 (1995)). On the other hand, it is known that the MgO barrier can be prepared by direct sputtering MgO from MgO target, or by vapor deposition of MgO by evaporating MgO shot (piece) utilizing electron beam.
As it is shown by the formula 1, infinitely large TMR value is expected where a ferromagnetic body of spin polarization of P=1 is used. The magnetic body of P=1 is called a half-metal. As a result of band calculation, oxides such as Fe3O4, CrO2, (La—Sr)MnO3, Th2MnO7, and Sr2FeMoO6, half-Heusler alloys such as NiMnSb, Full-Heusler alloys such as Co2MnGe, Co2MnSi, and Co2CrAl, each having an L21 structure are known the half-metals.
Currently, the MTJ device is practically applied as magnetic heads of hard disk and non-volatile random access magnetic memories (MRAMs). In a MRAM, MTJ devices are arranged in 2-dimensional matrix, and a magnetic field is applied by generating electric currents in independently disposed wirings. By this constitution, a high resistive state and a low resistive state are created by controlling magnetization of two magnetic layers of each MTJ element in parallel or in antiparallel, and thereby recording “one” and “zero” signals. Reading of the record is performed utilizing the TMR effect. In such a field of application, an MTJ device of low resistance is required for high-speed operation. In addition, an MTJ device of low resistance is required due to the increasing importance of spin transfer magnetization switching, where magnetization of the MTJ device is switched by injection of spin polarized current. Further, a technique to inject spins through a barrier into a semiconductor gains increasing importance in the field of Metal-Oxide-Semiconductor type field effect transistor (spin MOSFET) and spin transistors in which spin-dependent output power is obtained. In these fields, a barrier of low resistance is required so as to achieve large on-state current.
Under the above-explained circumstance, the conventional AlOx amorphous barrier is not appropriate for the spin electronics device because of a too large contact resistance, heterogeneous properties due to large interface roughness between the ferromagnetic layer and the barrier layer, and generally small TMR value of about several tens % at room temperature.
On the other hand, in an epitaxial tunnel junction utilizing the crystalline MgO barrier, the properties of its electronic structure results in appearance of coherent tunnel effect, where tunnel transmission of Δ1 electrons increases in ferromagnetic materials having bcc structure such as, for example, Fe and FeCo. Therefore, tunnel resistance decreases, and TMR value is largely enhanced from the value expected from formula 1 (for example, W. H. Butler et al., Phys. Rev. B 63, 054416 (2001)). Specifically, under theoretical calculation, excessively large value of 5300% is expected for MTJ having a structure of Fe/MgO (8 atomic layers)/Fe grown in (001) direction, and 34000% is expected for CoFe/MgO (8 atomic layers)/CoFe structure (X.-G. Zhang and W. H. Butler, Phys. Rev. B 70, 172407 (2004)).
Co-based full-Heusler alloy is an intermetallic compound having a composition of Co2YZ type, and is generally expected to be a half-metal, that is, expected to show P=1, in L21 structure or in B2 structure. The B2 or L21 structure can be obtained relatively easily by epitaxially growing an MgO barrier on a Co-based Heusler alloy film, and growing a Co-based Heusler alloy film on the MgO barrier. The Inventors have proposed a half-metal Heusler alloy Co2FeAlxSi1-x (0<x<1) with controlled Fermi level (WO2007/126071), and reported large TMR value at room temperature (R. Shan et al., Phys. Rev. Lett. 102, 246601 (2009)).
It has been theoretically pointed out that the coherent tunnel effect through the MgO layer is effective for Co-based full-Heusler alloy (Y. Miura et al., J. Phys.: Condens. Matter 19, 365228 (2007)). However, where the Co-based full Heusler alloy is used as a material of ferroelectric layer, tunnel junction of high quality cannot be obtained due to large lattice mismatches with MgO (for example, Co2FeSi and MgO shows mismatch of 6%), resulting in generation of defects such as dislocations in the MgO barrier (H. Sukegawa et al., Phys. Rev. B 79, 184418 (2009)). Specifically, Co-based full Heusler alloy grown on the MgO barrier tends to have a disordered structure, and a large TMR value expected from half-metal is not detected. In addition momentum in the direction perpendicular to the layer plane is not conserved due to generation of disordered structure in the interface. As a result, the theoretically simulated enhancement of TMR due to the coherent tunnel effect is not always observed.
In addition, it is required to apply bias voltage of several hundred mV to 1V in order to writing and reading of information using the MTJ device. However, in the MTJ device having the amorphous AlOx barrier or the MgO barrier, there has been a general problem of reduction of TMR value to half of the zero-bias value upon application of bias voltage of about 500 mV. The large bias-voltage dependence of the TMR value is mainly caused by lattice defects and interface roughness between the ferromagnetic layer and the barrier layer. In the conventional amorphous AlOx barrier or MgO having large lattice misfit, it is very difficult to improve the above-described bias voltage dependence.
The inventors have proposed to use MgAl2O4 having spinel structure as a barrier layer of MTJ so as to reduce lattice mismatch and to realize MTJ of improved quality, and practically realized a MTJ having MgAl2O4 barrier epitaxially grown with (001) plane orientation (R. Shan et al., Phys. Rev. Lett. 102, 246601 (2009); H. Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)). MgAl2O4 has a spinel structure (space group in International notation: Fd-3m) having a lattice constant of about 0.809 nm. Since half of the unit cell is smaller by ca. 4% compared with MgO (0.4213 nm), MgAl2O4 has excellent lattice matching with Fe and CoFe alloy each having bcc structure, and with the above-described Co-based Heusler alloy.
For example, lattice misfit can be controlled to small level, for example, 0.2% for MgAl2O4 and Fe, and 0.7% for MgAl2O4 and Co2FeAl0.5Si0.5. In the above-described Fe/MgAl2O4/Fe structure, in-plane lattice mismatch is about 1%, which is obviously smaller than lattice mismatch of about 3.5% between Fe and MgO (H. Sukegawa et al., Appl. Phys. Lett. 96, 212505 (2010)). The MgAl2O4 is a chemically stable material, which is, for example, known as a gem mineral, and does not have a deliquescence like MgO. Therefore, MgAl2O4 is promising as a material of high quality tunnel barrier. TMR value of the Fe/MgAl2O4/Fe structure is 117% at room temperature and is 165% at 15K. These TMR values are apparently larger than the values (TMR=38 to 51%) calculated from the formula 1 and spin polarization of Fe in the range of 0.40 to 0.45. In addition, there is an unknown advantage that bias-voltage dependence of TMR value is largely improved by reduction of lattice mismatch. However, the obtained TMR value was not sufficiently large in comparison with crystalline MgO, and improvement was required. In addition, large TMR value was only obtained by the composition of MgAl2O4, and lattice constant could not be changed continuously while maintaining the large TMR value.
Based on the above-described circumstances, an object of the present invention is to decrease the effective lattice constant of MgAl2O4 type insulator by disordering its crystal structure, and achieving a high TMR value that has not known in the prior art using the disordered structure as the tunnel barrier layer, and to realize continuous modulation of lattice constant while maintaining the high TMR value by controlling the element composition.